1. Field of the Invention
Disclosed is an improved acoustic downhole logging tool for measuring certain rock parameters indicative of selected properties of the sidewall material of a borehole, in both cased and in open holes. The tool is particularly useful in circumstances where the sidewall material is characterized by an acoustic velocity that is lower than the propagation velocity of an acoustic pulse in the borehole fluids.
2. Discussion of Related Art
Acoustic logging tools for measuring properties of the sidewall material of both cased and uncased boreholes are well known. Essentially such tools measure the travel time of an acoustic pulse propagating through the sidewall material over a known distance. In some studies, the amplitude and frequency of the acoustic pulse, after passage through the earth, are of interest.
In its simplest form, an acoustic logger consists of one or more transmitter transducers that periodically emit an acoustic pulse into the formation around the borehole. One or more receiver transducers, spaced apart by a known distance from the transmitter, hears the pulse after passage through the surrounding formation. The difference in time between pulse transmission and pulse reception divided into the distance between the transducers is the formation velocity. If the transducers do not contact the borehole sidewall, allowance must be made for time delays through the borehole fluid.
Throughout this disclosure, the term "velocity", unless otherwise qualified, shall be taken to mean the velocity of propagation of an acoustic wavefield through an elastic medium. The term does not mean the velocity of motion of a medium.
Acoustic wavefields propagate through elastic media in different modes. The modes include: Compressional or P-waves, wherein particle motion is in the direction of wave travel; transverse shear or S-waves, which, assuming a homogeneous, isotropic medium, may be polarized in two orthogonal directions, with motion perpendicular to the direction of wave travel; Stonley waves, which are guided waves that propagate along the fluid-solid boundary of the borehole; and compressional waves that propagate through the borehole fluid itself. There also exist asymmetrical flexural waves as will be discussed later.
P-waves propagate through both fluids and solids. Shear waves cannot exist in a fluid. Compressional waves propagating through the borehole fluid may be mode-converted to shear waves in the borehole sidewall material by Snell's law refraction provided the shear-wave velocity of that material is greater than the compressional-wave velocity of the borehole fluids. If that is not true, then shear waves in the sidewall material can be generated only by direct excitation.
Among other parameters, the various modes of propagation are distinguishable by their relative velocities. The velocity of compressional and transverse shear waves is a function of the elastic constants and the density of the medium through which the waves travel. The S-wave velocity is, for practical purposes, about half that of P-waves. Stonley waves may be somewhat slower than S-waves. Compressional wavefields propagating through the borehole fluid are usually slower than formational shear waves but for boreholes drilled into certain types of soft formations, the borehole fluid velocity may be greater than the sidewall formation S-wave velocity. The velocity of flexural waves is said to approach the S-wave velocity as an inverse function of the acoustic excitation frequency. Some authors refer to flexural waves as pseudo-Raleigh waves.
In borehole logging, a study of the different acoustic propagation modes provides diagnostic information about the elastic constants of the formation, rock texture, fluid content, permeability, rock fracturing, the goodness of a cement bond to the well casing and other data. Typically, the output display from an acoustic logging tool takes the form of time-scale recordings of the wave train as seen at many different depth levels in the borehole, each wave train including many overlapping events that represent all of the wavefield propagation modes. For quantitative analysis, it is necessary to isolate the respective waveform modes. S-waves are of particular interest. But because the S-wave arrival time is later than the P-wave arrival time, the S-wave event often is contaminated by later cycles of the P-wave and by interference from other late-arriving events. Therefore, known logging tools are designed to suppress undesired wavefields either by judicious design of the hardware or by post-processing using suitable software.
In one form of tool, the transmitter and receiver transducers are mounted in pads that contact the sidewall of the borehole. Both P-waves and S-waves are generated but not other undesirable waves. The P-waves are gated out by an electronics package, leaving only the S-waves. The problem with that arrangement is not only wear on the pads and excess friction when the tool is drawn up the borehole, but also the frictional road noise that is generated by the passage of the tool through the borehole. Therefore, modem tools are centered in the borehole and transmit acoustic pulses through the drilling fluid into the sidewall by refraction.
R. L. Caldwell, in U.S. Pat. No. 3,333,328, issued Jul. 25, 1967, teaches use of a tool that is suspended centrally in the borehole, separated from the sidewall. He employs cylindrical transducers to generate and to receive S-waves by refraction at the borehole-wall interface. To avoid acoustic interference with other arrivals, he employs delay-gating to preferentially isolate the desired signals such as S-waves.
In U.S. Pat. No. 4,813,028, issued Mar. 14, 1989 to O. Y. Liu, there is described an acoustic well logging apparatus that utilizes a rare earth acoustic cylindrical transducer to provide low frequency acoustic energy within the borehole so that characteristics of subsurface formations may be obtained. The parameters of formation permeability are determined by measuring the attenuation of Stonley waves produced by the transducer. It is of interest that Liu recognizes the existence of flexural waves but complains that those waves interfere with the desired Stonely-waves and he seeks to suppress the flexural waves.
Many of the later workers in the art prefer to use flat-plate, bender bar transducers as being capable of providing direct excitation of flexural waves in the borehole sidewall. By so doing, the problem of mode conversion by refraction (or the lack thereof) in slow formations is mitigated.
J. Zemanec, in U.S. Pat. No. 4,516,228, issued May 7, 1985 provides a borehole logging system that employs a compressional wave transmitter and a direct-excitation shear wave transmitter. The transmitters are alternately fired to impart compressional and shear waves in the surrounding borehole formations. A single bender-bar receiver, spaced apart from the transmitter in the borehole is alternately gated so that the voltages across its pair of piezoelectric planar surfaces are subtracted during the expected period of compressional wave output and added during the expected arrival time period of asymmetrical motion of the receiver to provide shear wave output. It is of interest that the bender-bar receiver transducer was isolated from the logging sonde by a soft supporting pad.
F. A. Angonna et al. in U.S. Pat. No. 4,649,525, issued Mar. 10, 1981 disclose an acoustic logger that employs a bender-type transducer as a point source of an acoustic shear wave. The bender transducer includes opposed unrestricted planar surfaces mounted within a liquid-filled compartment within the tool. The surfaces of the transducer are emplaced longitudinally along the axis of the tool and exposed to the coupling liquid. One or more bender bars may also be used as receivers. The active faces of the receivers are oriented substantially in the same direction as is the active surface of the transmitter. As with the previous patent, the transducers are resiliently supported on soft mounts.
Another acoustic logging sonde that employs a bender-type transducer is described in U.S. Pat. No. 4,782,910, issued Nov. 8, 1988 to C. C. Shaw. A bender-bar transducer has a flat piezoelectric element secured to a fiat elongated inert element. The ends of the inert element are hinged on a supporting rectangular frame inside a rectangular opening therein. The sides of the active and inert elements are exposed so that when a voltage is applied, a dipole acoustic wave is generated by the transducer. The frame serves as a reaction mass. The assembly including frame and elements are resiliently suspended by rubber straps within a compartment in the sonde, thereby to acoustically isolate the frame and active elements from the sonde. In one embodiment, dual piezoelectric elements are secured to opposite sides of the inert element. The edges of the frame serve as baffles to acoustically separate the two exposed surfaces of the active elements.
Vogel et at. in U.S. Pat. No. 4,834,209, issued May 30, 1989 disclose a transverse wave logging tool that consists of a plurality of sets of transducers that are mounted around a cylindrical mandrel. One set of four transducers acts as a set of transmitters; a second set of four transducers acts as a set of receivers. The active faces of the transducers are characterized by two orthogonal dimensions, one of which is a half wavelength long relative to the acoustic excitation energy applied to the transmitter transducer and the transverse-wave formation velocity. The transmitter transducer generates transverse S-waves in the formation by direct excitation of the borehole sidewall along the normal thereto rather than by critical-angle refraction. Receiver transducers detect converted-compressional waves resulting from transverse waves that were generated by the acoustic excitation energy.
A transducer employing piezoresistive elements is taught in U.S. Pat. No. 4,949,316, issued Aug. 14, 1990 to K. W. Katahara. The active element may be a flat silicon plate upon which the piezoresistors are formed. The plate is supported from its ends by springs that are secured within an oil-filled compartment in a sonde.
J. Zemanec et at., in a paper published in The Log Analyst for May/June, 1991, discusses shear wave logging using multiple sources. He explains the concept of direct excitation and the generation of flexural waves by a dipole transmitter. He illustrates the difference between asymmetrical dipolar flexural waves and omni-directional compressional waves that are generated by a monopolar transmitter. Graphs are presented showing the dependence of the group and phase velocities on signal frequency.